code: def test_expression_function_string():
      # expr() can only work with str
      er = expr("2 * 2")
      assert isinstance(er, ExprRef)
      assert repr(er) == "ExprRef({\n  expr: '2 * 2'\n})"
  
file_path: altair/tests/expr/test_expr.py
imports: import operator
  import pytest
  from altair import expr
  from altair import datum
  from altair import ExprRef
  from jsonschema.exceptions import ValidationError
label_desc: Use image node_id 12 for calling a global function with example usage: test_expression_function_string() without return types